The present invention generally relates to a level control circuit, and more specifically, to a level control circuit capable of providing a wide-range level control function in a portable radio communication apparatus.
In portable radio communication apparatuses, levels of received radio signals are considerably varied, depending upon the present locations of the portable radio communication apparatuses, and the level variation widths reach to 80 dB. To supply such largely varied radio signals to demodulators within a level of preselected range so as to demodulate information from the received radio signals, a level control circuit capable of controlling a signal level in such a wide range is necessarily required.
In this sort of level control circuit, a variable attenuator is employed. However, since the coupling in a higher frequency band may readily occur in both input and output terminals of the variable attenuator in the radio frequency band and the intermediate frequency band, such a wide-range level control such as 80 dB can be hardly performed only by a single variable attenuator.
As represented in FIG. 1, the Applicants have studied that a required attenuation amount could be achieved by two sets of variable attenuators during the stages to conceive the inventive idea of the present invention. That is, FIG. 1 schematically represents an RF-IF converting unit employed in a receiver unit of a portable radio communication apparatus. The RF-IF converting unit converts an RF signal S1 received by an antenna into an intermediate frequency band signal S2 having a constant average level. This RF-IF converting unit of FIG. 1 is arranged by two sets of variable attenuators 1, 2 cascade-connected to each other, the respective attenuation amounts of which are controlled in response to a commonly used level control signal C1; a converter 3 for converting a level-controlled RF signal derived from these variable attenuators 1, 2 into an intermediate frequency (IF) band signal; and a bandpass filter 4 for extracting a desired intermediate frequency band signal S2 from the output signal of the converter 3.
With this circuit arrangement, the level control could be achieved in a wider range than that achieved by a single variable attenuator. However, another problem of the coupling in a higher frequency band is newly produced between the respective variable attenuators. Thus, each of these variable attenuators should be made in a shield structure.
Nevertheless when such a shield structure is employed, the resulting radio communication apparatus would become bulky, which could not satisfy a need to make a compact portable type radio communication apparatus.
Although the above-described coupling in a higher frequency band via a space between the respective variable attenuators can be prevented by employing the shield structure, another coupling via a level control signal line between them could not be prevented.